A forum for the exchange of circuits, systems, and software for real-world signal processing
Volume 38, Number 4, 2004
In This Issue
Editors’ Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Wideband CMOS Switches (Ask The Application Engineer—34) . . . . . . . . . . . . . . . . . . . . 3
Support for the Designer—Improved ADI Website Helps You . . . . . . . . . . . . . . . . . . . . . . . 8
Designing Efficient, Real-Time Audio Systems with VisualAudio™ . . . . . . . . . . . . . . . . . 11
Recent Product Introductions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Editors’ Notes
You’ve opened the book on the final
quar terly issue of 2004, our 38 t h
sequential year in print—and 6th online,
at analog.com/analogdialogue. Perhaps
you’ve read all four issues cover to cover.
Or perhaps this is your first acquaintance
with Analog Dialogue. In any event, here’s
your opportunity to spend a moment to be
tempted to read an article you may have
missed—or to contemplate a title that
you’ve already read. You can find copies of all these issues online
in the archives at http://www.analog.com/library/analogdialogue/
archives.html
SATELLITE RADIO, MP3s, AND STREAMING AUDIO
Back in the days of analog LPs on vinyl,
I owned over 500 record albums. Then,
as an early adopter of compact discs, I
bought all of my new music on CDs,
and even started to replace some of my
records. Soon I abandoned the turntable
altogether and gave all of my albums to
my brother. Sadly, a f lood ruined all the
records, but I shed nary a tear, exulting
in the luxur y of the newer, smaller,
virtually indestructible CDs.
The year started—in Number 1—with PID control algorithms,
fan- speed in temperature control, and video technology in
automotive safety. In this column, you could have read a
rambling historical discourse on (mostly—but not entirely—
analog) multipliers.
Yet lately I’ve realized that I rarely buy CDs anymore—and
when I listen to them it’s almost always in the car. In my family
room I usually listen to one of the dozens of commercial-free,
CD - quality audio channels that are available over the digital
cable. It makes available a much wider variety of music, and
lets me view trivia, history, and other information on the song,
album, and artist. When at the computer, I listen to streaming
audio from one of several providers, using one of the available
media players. The small annual payment for this service
makes it possible to listen to high- quality audio from over
1,000 stations and lets me download my favorite songs for a
nominal additional fee.
In the following issue—Number 2—you (could have) read about
current measurement in solenoids for automotive controls, bridge
amplification with digitally programmed gain and offset, and
practical techniques to avoid op-amp instability due to capacitive
loading. There was also a description of techniques we use for
in-package trimming of a low-cost CMOS amplifier with wide
bandwidth, offsets less than 65 V and drifts less than 7 V/ C.
T he penu lt i mate issue —Nu mber 3 — had a n “A sk T he
Applications Engineer” (#33) feature on direct digital synthesis
(DDS), plus articles on JPEG2000 image compression and
a digitally adjustable cable equalizer. You also could have
read about a reader’s discover y—in a NASA vehicle, in
equipment designed before he was born—of an ingenious but
deceptively simple hot-wire anemometer. Its design principle
was described in a (still interesting) article on measuring
f luid f low with a self- balancing bridge; originally appearing
in our Volume 5, in 1971; it was reprinted in this issue.
And in these pages today you can read about our designer-oriented
updated website, a new software tool for memory-efficient, realtime audio designs, and an “Ask The Applications Engineer” (#34)
on wideband CMOS switches.
Thus we close the book on Volume 38 and look forward eagerly to
Volume 39, which will commemorate Analog Devices’s 40th year
of providing the electronic industry with innovative products,
guidance, and ideas for analog- and digital real-world signal
processing solutions.
Dan Sheingold [dan.sheingold@analog.com]
www.analog.com/analogdialogue
dialogue.editor@analog.com
Analog Dialogue is the free technical magazine of Analog Devices, Inc., published
continuously for 38 years—starting in 1967. It discusses products, applications,
technology, and techniques for analog, digital, and mixed-signal processing. It is
currently published in two editions—online, monthly at the above URL, and quarterly
in print, as periodic retrospective collections of articles that have appeared online. In
addition to technical articles, the online edition has timely announcements, linking to
data sheets of newly released and pre-release products, and “Potpourri”—a universe
of links to important and rapidly proliferating sources of relevant information and
activity on the Analog Devices website and elsewhere. The Analog Dialogue site is,
in effect, a “high-pass-filtered” point of entry to the www.analog.com site—the
virtual world of Analog Devices. In addition to all its current information, the
Analog Dialogue site has archives with all recent editions, starting from Volume 29,
Number 2 (1995), plus three special anniversary issues, containing useful articles
extracted from earlier editions, going all the way back to Volume 1, Number 1.
If you wish to subscribe to—or receive copies of—the print edition, please go to
www.analog.com/analogdialogue and click on <subscribe>. Your comments
are always welcome; please send messages to dialogue.editor@analog.com
or to these individuals: Dan Sheingold, Editor [dan.sheingold@analog.com]
or Scott Wayne, Managing Editor and Publisher [scott.wayne@analog.com].
At the gym, at the beach, or in the backyard, I listen to my MP3
player. Where do the MP3s come from? Most were ripped from
my CD collection, but the newest ones are all downloads. Why
buy the whole CD when I need buy only my favorite songs—for
a fraction of the cost—and eliminate the storage problem at
the same time.
In the car, I listen mostly to the radio, but am constantly
annoyed and frustrated by the large number of commercials,
especially at drive time. Although I listen to CDs in the car,
those jewel boxes take up too much space and are too hard
to open safely while driving. CDs in sleeves are more space
efficient and are easier to handle, but they’re sometimes hard
to identify without their covers. An FM modulator lets me
listen to my MP3 player in the car, but the audio quality is not
as good as a CD, and it’s sometimes difficult to find an unused
radio frequency in Boston’s busy metropolitan market.
Thus, one option that now tops the priority list for my new car
is satellite radio, either XM or Sirius. In the early days of cable,
skeptics wondered why people would pay to watch TV when
they could watch it for free. Today, many people feel the same
way about radio, but I look forward to the day when I can give
my CD collection to my brother and rely on streaming media
wherever I go.
Why am I writing about this here? Because as I drive to work
each day I can feel proud that Analog Devices offers amplifiers,
converters, and processors that enable satellite receivers, set-top
boxes, computer audio, and MP3 players to be small, flexible,
power- efficient, and inexpensive—all the while providing
high quality and functionality—plus the software that helps
developers to quickly bring these products to market.
Your comments are welcome.
ISSN 0161-3626 ©Analog Devices, Inc. 2005
Scott Wayne [scott.wayne@analog.com]
Ask The Application Engineer—34
Wideband CMOS Switches
By Theresa Corrigan [theresa.corrigan@analog.com]
Q: You mention off isolation and insertion loss. Could you explain what
these are?
A: Yes, the two most important parameters that describe the
performance of an RF switch are the insertion loss in the closed
state and the isolation in the open state.
Q: What is a CMOS wideband switch?
Off isolation is defined as the attenuation between input and
output ports of the switch when the switch is off. Crosstalk is a
measure of the isolation from channel to channel.
A: CMOS wideband switches are designed primarily to meet the
requirements of devices transmitting at ISM (industrial, scientific,
and medical) band frequencies (900 MHz and up). The low
insertion loss, high isolation between ports, low distortion, and
low current consumption of these devices make them an excellent
solution for many high frequency applications that require low
power consumption and the ability to handle transmitted power
up to 16 dBm. Examples of applications mentioned later in this
article, include car radios, antenna switching, wireless metering,
high speed filtering and data routing, home networking, power
amplifiers, and PLL switching.
A: To improve their bandwidth, wideband switches use only
N- channel MOSFETs in the signal path. An NMOS-only
switch has a typical –3-dB bandwidth of 400 MHz—almost
twice the bandwidth performance of a standard switch with
NMOS and PMOS FETs in parallel. This is a result of the
smaller switch size and greatly reduced parasitic capacitance
due to removal of the P- channel MOSFET. N- channel
MOSFETs act essentially as voltage- controlled resistors.
The switches operate as follows:
0
VDD = 1.65V TO 2.75V
TA = 25C
–10
–20
–30
ISOLATION (dB)
Q: How do these switches come to be so much faster than typical analog
CMOS switches?
For example, the ADG919 SPDT switch provides about
37 dB of isolation at 1 GHz, as shown in Figure 2. The
same device, using the chip-scale package (CSP)—offered
for space-constrained wireless applications, such as antenna
switching—offers a 6-dB improvement (43 dB at 1 GHz).
–40
–50
S12
–60
–70
–80
S21
–90
–100
10k
Vgs > Vt Æ Switch ON
100k
As the signal frequency increases to greater than several
hundred megahertz, parasitic capacitances tend to dominate.
Therefore, achieving high isolation in the switches’ off -state
and low insertion loss in the on state for wideband applications is
quite a challenge for switch designers. The channel resistance
of a switch must be limited to less than about 6 ohms to achieve
a low-frequency insertion loss of less than 0.5 dB on a line
with 50-ohm matched impedances at the source and load.
As a departure from the familiar switch topology, inserting
a shunt path to ground for the off -throw—and its associated
stray signal—allows the design of switches with increased offisolation at high frequencies. The FETs have an interlocking
finger layout that reduces the parasitic capacitance between
the input (RFx) and the output (RFC), thereby increasing
isolation at high frequencies and enhancing crosstalk rejection.
For example, when MN1 is on to form the conducting path
for RF1, MN2 is off and MN4 is on, shunting the parasitics at
RF2 to ground, as shown in Figure 1.
RF COMMON
MN3
MN2
R1
R3
100M
1G
10G
RF2
To obtain the best insertion-loss performance from the ADG9xx
family of switches, one should operate the part at the maximum
allowable supply voltage of 2.75 V. The reason can be seen in
Figure 3, which shows plots of insertion loss versus frequency for
the ADG919 at three different values of supply voltage.
–0.30
–0.35
–0.40
VDD = 2.75V
VDD = 2.5V
–0.45
–0.50
–0.55
VDD = 2.25V
–0.60
–0.65
–0.75
MN4
IN
Figure 1. A typical transistor based Tx/Rx switch.
Analog Dialogue Volume 38 Number 4
Insertion loss is the attenuation between input and output ports of
the switch when the switch is on. The switch is generally one of
the first components encountered in a receiver’s signal path, so a
low insertion loss is required to ensure minimum signal loss. Low
switch insertion loss is also important for systems that require a
low overall noise figure.
–0.70
R2
R4
Figure 2. Off isolation vs. frequency.
INSERTION (dB)
Where Vgs is the gate-to-source voltage and Vt is defined as the
threshold voltage—above which a conducting channel is formed
between the source and drain terminals.
RF1
10M
FREQUENCY (Hz)
Vgs < Vt Æ Switch OFF
MN1
1M
–0.80
10k
TA = 25C
100k
1M
10M
100M
1G
10G
FREQUENCY (Hz)
Figure 3. Insertion loss vs. frequency.
3
Q: How does insertion loss relate to the On-resistance spec of a standard
analog switch?
A: Signal loss is essentially determined by the attenuation
introduced by switch resistance in the on condition, Ron , in
series with the source-plus-load resistance—measured at
the lower frequencies of operation. Figure 4 shows a typical
profile of on -resistance as a function of source voltage for an
N-channel MOSFET device.
28
24
RON ()
Q: How about the ESD (electrostatic discharge) performance as
compared to GaAs?
A: The ADG9xx family of parts passes the 1-kV ESD HBM
(human body model) requirement. ESD protection circuitry is
easily integrated on these CMOS devices to protect the RF and
digital pins. This makes the switches ideal for any applications
that are ESD sensitive, and they offer a reliable alternative to
GaAs devices having ESD ratings as low as 200 V.
Q: What are the other important specifications of these switches?
20
A: Video Feedthrough (Figure 5) is the spurious dc transient
present at the RF ports of the switch when the control voltage
is switched from high- to- low-, or low- to- high, without an
RF signal present. This is analogous to charge injection of a
typical analog switch. It is measured in a 50-ohm test setup,
with 1-ns (rise-time) pulses and a 500-MHz bandwidth.
16
12
8
4
low insertion loss (0.5 dB) all the way down to dc. In addition to
providing a smaller, more efficient design solution, the ADG9xx
family is less power-demanding, consuming less than 1 A over
all voltage and temperature conditions.
T
0
0.4
0.8
1.2
1.6
2.0
2.4
1
VS (V)
Figure 4. On resistance vs. source voltage.
Q: What technologies have been commonly used in the design of highfrequency switches?
CTRL
A: Traditionally, only a few processes were available for developing
good wideband/RF switches. Gallium arsenide (GaAs) FETs,
PIN diodes, and electromechanical relays have dominated the
market, but standard CMOS is now a strong entry.
PIN diodes are highly linear devices with good distortion
characteristics, but they have many drawbacks given today’s
high performance demands. They have very slow switching
times (microseconds, compared to nanoseconds for CMOS
switches); they are power-hungry, making them unsuitable for
many battery-operated devices; and—unlike CMOS switches
with their response from RF to dc—there is a practical lower
frequency limit to the use of PIN diodes as linear switches.
GaAs has been popular because of its low on resistance, low
off capacitance, and high linearity at high frequencies. As
CMOS process geometries continue to shrink, however, the
performance of CMOS switches has increased to the extent
that they can achieve –3-dB frequencies of up to 4 GHz and
are able to compete with GaAs switches. Designed to maximize
bandwidth while maintaining high linearity and low power
consumption, CMOS switches now offer a practical alternative
to GaAs switches in many low-power applications.
RFC
2
CH2 p-p
2.002mV
CH1 500mV
Q: What does this mean?
A: It means that if the insertion loss at 1 GHz was 0.8 dB with a
low-level input, it would be 1.8 dB with a 17-dBm input signal
[Note: dBm is the dB (logarithmic) measure of the ratio of
power to 1 mW, or voltage to 224 mV in 50 ohms. 17 dBm
corresponds to 50 mW, or 1.6 V rms or 4.5 V p-p].
20
4
18
16
14
P–1dB (dBm)
GaAs switches, as such, need dc-blocking capacitors in series with
the RF ports, effectively floating the die relative to dc ground, so
that the switches can be controlled with positive control voltages.
Wideband switches, such as the ADG9xx family, do not have
this requirement, eliminating concerns of reduced bandwidth,
the impact of the capacitors on overall system performance, and
the extra space and cost of GaAs solutions. Eliminating the
blocking capacitors allows the ADG9xx parts to maintain their
M10.0ns
P1dB (1-dB compression point) is the RF input power level at
which the switch insertion loss increases by 1 dB over its low-level
value. It is a measure of the RF power-handling capability of the
switch. As shown in Figure 6, the ADG918 has a P1dB of 17 dBm
at 1 GHz, with V DD = 2.5 V.
Q: So what are the main benefits of CMOS wideband switch solutions
over gallium arsenide?
A: Switches, such as the ADG9xx family of parts, have an
integrated TTL driver that allows easy interfacing with other
CMOS devices, since CMOS is compatible with LVTTL logic
levels. The small size of devices with integrated drivers is a
solution for many space-constrained applications.
CH2 1mV 
Figure 5. Video feedthrough.
12
10
8
6
4
VDD = 2.5V
TA = 25C
2
0
0
250
500
750
1000
1250
1500
FREQUENCY (MHz)
Figure 6. 1-dB compression point vs. frequency.
Analog Dialogue Volume 38 Number 4
Q: Power-handling capability seems to decrease substantially at the
lowest frequencies in Figure 6. Why?
A: In normal operation, the switches can handle a 7-dBm (5-mW)
input signal. For a 50-ohm load, this corresponds to a
0.5-V rms signal, or 1.4 V peak-to-peak for sine waves.
[V p-p = V rms  2  ÷2].
The power-handling capability is reduced at lower frequencies
for two reasons:
50
VG
VS
N+
VD
Both of the above mechanisms can be overcome by applying a small
dc bias (about 0.5 V) to the RF input signal when the switch is
being used at low frequencies (<30 MHz) and high power—greater
than 7 dBm (or 5 mW, 1.4 V p-p in 50 ohms). This will raise the
minimum level of the sine-wave input signal and thus ensure
that the parasitic diodes are continually reverse-biased and that
the shunt transistor, never seeing Vgs > Vt, remains in the off state
for the whole period of the input signal. Figure 9 again shows a
plot of input- and output signals at 100 MHz and 10 dBm input
power (about 2 V p-p in 50 ohms), but this time with a 0.5-V dc
bias. It is clearly visible that clipping or compression no longer
occurs at 100 MHz.
50
N+
P TYPE
SUBSTRATE
Figure 7. Physical NMOS structure.
T
As shown in Figure 7, the inherent NMOS structure consists of two
regions of N-type material in a P-type substrate. Parasitic diodes
are thus formed between the N and P regions. When an ac signal,
biased at 0 V dc, is applied to the source of the transistor, and Vgs
is large enough to turn the transistor on (Vgs > Vt), the parasitic
diodes can be forward-biased for some portion of the negative halfcycle of the input waveform. This happens if the input sine wave
goes below approximately –0.6 V, and the diode begins to turn
on, thereby causing the input signal to be clipped (compressed),
as shown in Figure 8. The plot shows a 100-MHz, 10-dBm input
signal and the corresponding 100-MHz output signal. It is readily
seen that the output signal has been truncated.
REF1 FREQ
99.98MHz
REF1 AMPL
1.85V
T
REF1 FREQ
99.98MHz
REF1 AMPL
1.85V
C1 FREQ
100.05MHz
C1 AMPL
1.51V
C1 FREQ
100.00MHz
C1 AMPL
1.75V
CH1 500mV 
M2.00ns
CH1
0V
Figure 9. 100-MHz, 10-dBm input/output signals
with 0.5-V dc bias.
Q: How do I apply a dc bias to RF inputs?
A: To minimize any current drain through the termination
resistance on the input side, it is best to add the bias on the
output (RFC) side. This is the best practice, especially for
low-power portable applications, but it may be necessary to
apply dc-blocking capacitors on the RF outputs if downstream
circuitry cannot handle the dc bias.
Q: Can these switches operate with a negative supply?
A: They can operate with a negative signal on the GND (ground)
pin as long as it adheres to the –0.5 V to +4 V Absolute
Maximum Rating for V DD to GND. Note that operating the
part in this manner places the internal terminations at this new
GND potential—an undesirable effect in some applications.
Q: What about the distortion performance of these switches?
CH1 500mV 
M2.00ns
CH1
0V
Figure 8. 100-MHz, 10-dBm input/output signals
with 0-V dc bias.
At low frequencies, the input signal is below the –0.6 V level for
longer periods of time, and this has a greater impact on the 1-dB
compression point (P1dB).
The second reason why parts can handle less power at lower
frequencies is the partial turn-on of the shunt NMOS device
when it is supposed to be off. This is very similar to the mechanism
described above where there was partial turn-on of the parasitic
diode. In this case, the NMOS transistor is in the off state, with
Vgs < Vt. With an ac signal on the source of the shunt device,
there will be a time in the negative half-cycle of the waveform
where Vgs > Vt, thereby partially turning on the shunt device.
This will compress the input waveform by shunting some of its
energy to ground.
Analog Dialogue Volume 38 Number 4
A: When tones at closely spaced frequencies are passed through
a switch, the nonlinearity of the switch causes false tones to be
generated, causing undesired outputs at other frequencies. In
communications systems, where channels are becoming more
tightly spaced, it is essential to minimize this intermodulation
distortion (IMD) to ensure minimum interference. Applying
two closely spaced equal-power signals with a set frequency
spacing (e.g., 900 MHz and 901 MHz) to the input of a device
under test (DUT), results in the output spectrum shown in
Figure 10. The 3rd -order harmonic, usually expressed in dBc,
is the log of the ratio of the power in the 3rd order harmonic
to the power of the fundamental. The larger the (negative)
value, the lower the distortion. Sending these tones through
the ADG918, using a combiner with an input power of 4 dBm,
resulted in an IP3 of 35 dBm as shown in Figure 11. [Note: an
excellent discussion of various types of distortion can be found
in “Ask The Applications Engineer—13”]1
5
Q: What is a reflective switch?
INTERMODULATION DISTORTION (dB)
0
A: The ADG902 (SPST), ADG919 (SPDT), ADG936R (dual
SPDT), and the ADG904R (SP4T) parts are described as
reflective switches because they have 0-ohm shunts to ground.
–5
FUNDAMENTAL
–10
–15
Q: Where would I use an absorptive switch over a reflective switch?
–20
IMD
PRODUCT
–25
A: An absorptive switch has a good impedance-match, or voltage
standing-wave ratio (VSWR), on each port, regardless of the
switch mode. It should be used when there is a need for proper
back-termination in the off channel, to maintain a good VSWR.
An absorptive switch is therefore ideal for applications that
require minimum reflections back to the RF source. It also
ensures that the maximum power is transferred to the load in
a 50-ohm system.
–30
–35
–40
–45
–50
2f1 f2
f1
f2
2f2 f1
A reflective switch is suitable for applications where high off -port
VSWR does not matter and the switch has some other desired
performance feature. Reflective switches are commonly used
in applications where the matching is provided elsewhere in
the system. In most cases, an absorptive switch can be used
instead of a reflective switch, but not vice versa.
FREQUENCY
Figure 10. Output spectrum of two-tone IMD test.
40
35
Q: How can I determine the VSWR of these switches?
30
A: VSWR—voltage standing-wave ratio—the ratio of the sum
of forward and reflected voltages to the difference of forward
and reflected voltages—indicates the degree of impedance
match present at the switch RF port. When it comes to
measurement, it is easier to describe the impedance match in
terms of return loss, the amount of reflected power relative to
the incident power at a port.2
IP3 (dBm)
25
20
15
10
5
0
250
VDD = 2.5V
TA = 25C
350
450
550
650
750
850
FREQUENCY (MHz)
Figure 11. IP3 vs. frequency.
IP3—Third-order intercept point. The IMD is measured, and
from this the IP3 value is calculated. IP3 is a figure of merit—in dBm—
for the device. IP3, specified in the data sheet, is a measure of the
distortion caused by the switch due to the power in these false tones.
The larger the IP3 value the smaller the tones in the adjacent channels,
indicating that the switch has good harmonic performance.
Simply by measuring both incident and reflected power, the
return loss can be determined, and from this the VSWR can
be calculated by using readily available VSWR/return-loss
conversion charts. Figure 13 shows a typical return-loss
curve for the ADG918 in the on - and off conditions. Note
that the ADG918, an absorptive switch, has good return-loss
performance for the off, as well as the on, switch. The ADG919
version, which does not include termination resistors, would
not have good return-loss performance in the off condition.
0
–5
Q: What configurations are available in the ADG9xx family?
Q: What is an absorptive switch?
RF1
RFC
RF2
RF2
CTRL
CTRL
50
Figure 12. ADG918, an absorptive switch, and ADG919,
a reflective switch.
6
–20
OFF SWITCH (ADG918)
–25
ON SWITCH
–35
–40
10k
100k
1M
10M
100M
1G
10G
FREQUENCY (Hz)
Figure 13. Return loss vs. frequency for the ADG918 switch.
ADG919
RF1
50
–15
–30
A: The ADG901 (SPST), ADG918 (SPDT), ADG936 (dual
SPDT), and the ADG904 (SP4T) parts are described as
absorptive (matched) switches, because they have on-chip
50-ohm-terminated shunt legs.
RFC
RETURN LOSS (dB)
–10
A: The ADG9xx family comprises SPST (single- pole, singlethrow), SPDT (single-pole, double-throw), and dual -SPDT
switches—and 4:1 single-pole multiplexers (SP4T). These are
offered in both absorptive and reflective versions, in order to
suit all application needs.
ADG918
TA = 25C
VDD = 2.5V
Q: Now that you’ve explained how these parts perform, tell me where
and how they are used.
A: Due to their low insertion loss at up to 1-GHz and wide –3-dB
bandwidth (up to 4 GHz), switches in this family are ideal for
many automotive entertainment systems.
They have found homes in tuner modules and set-top
boxes to switch between the cable-TV input and the off-air
antenna input. Another area where these parts are suitable
is in car-radio antenna switching. Because these are
Analog Dialogue Volume 38 Number 4
generally 50 -ohm-impedance systems, the 50-ohm internal
terminations offered by the absorptive version of these
switches—the ADG901, ADG918, and ADG904—ensure
excellent impedance-matching and minimum reflections.
The variety of topologies available makes these parts very
easy to design into antenna-diversity-switch applications,
allowing the user to switch between several antennas and a
single tuner in multiband radios.
Q: What is PLL Switching, and why use the ADG918?
A: Switching between two phase-locked loops (PLLs)—commonly
described as the ping-pong technique—allows a designer to
achieve faster system settling times. The low power-consumption
and simple single-pin control of the ADG918 make it an easy
solution to integrate.
In switching between two oscillators, the desired isolation
performance can be achieved by cascading—i.e., connecting
a number of switches in cascade. This is a very simple way to
provide a high-isolation specification for a system, preventing
any interference at the higher frequencies. Cascading five
ADG918s provides 130-dB isolation at 1 GHz, with an insertion
loss of 3 dB. In this application, such an increase in insertion
loss is not material, since the principal concern is about the
signal levels relative to one another.
These parts are also suitable for wireless metering systems,
providing the required isolation between transmit and receive
signals (Figure 14).
LNA
ANTENNA
ADG918
Tx/Rx SWITCH
A nice feature of the ADG918 in this application is that it acts
as an integrated low-pass filter, eliminating the unwanted
harmonics created by the two PLLs. Achieved by the natural
increase in insertion loss at high frequencies, it easily prevents
the unwanted harmonics from propagating through the
switches, as shown in Figures 15 and 16.
PA
Figure 14. Tx/Rx switching.
These parts are perfect for high-speed filter selection
and data routing: the ADG904 can be used as a 4:1
demultiplexer to switch high -frequency signals between
different filters—and also to multiplex the signal to the
output. For differential filter selection and data routing,
the ADG936 dual SPDT (single - pole, double -throw) switch
is an ideal solution. Data switching in modem cards for
point-to -point wireless systems, such as microwave
radio links for military and avionic applications, requires
the high-frequency performance offered by the ADG9xx
family of parts.
PLL 1
SWITCH
PLL 2
Figure 15. PLL switching application.
GaAs SWITCH
They are also suitable for home-networking applications—
systems allowing the wireless remote control of many different
functions, such as opening and closing roller blinds, control
of lighting (on, off or dimming)—in which the information is
transmitted through a wireless link. The excellent isolation
performance at high frequency and low power-consumption
preserve a system’s current budget—thus constituting an
ideal application.
Due to their high frequency range—up to 4 GHz—this family
of parts are also suitable for many Bluetooth ® technologies—
enabling wireless communication in the 2.5 - GHz ISM
frequency band.
Wideband switches can be used in the design of power
amplif iers (PAs) with 800 -, 900 -, 1900 -, 2100 -MHz
frequencies—for cellular CDMA and GSM applications.
The switch is used in the feed -forward correction loop
around the main amplif ier, allowing the active - and
passive feedback- and feed -forward paths to be switched
out, permitting the amplif ier’s distortion levels to be
tested. The switch allows for gain - and phase correction
in the system. The high isolation, low insertion loss, and
the low distortion at 900 MHz make the ADG9xx family
ideal for PA design in this frequency range.
The ADG918 can be used to implement PLL switching for
frequency-hopping in GSM applications.
O/P
ADG9xx
f
2f
3f
Figure 16. ADG918 switch cascade acting as integrated
low-pass filter, compared with naked GaAs switching.
Q: So ... to summarize?
A: In summary, CMOS wideband switches, especially those in
the ADG9xx family, are excellent choices for all applications
in the ISM band that require high isolation and low insertionloss for battery- operated devices with space constraints.
Evaluation kits are available from Analog Devices to make
the design-in of these parts fast and hassle free—every
designer’s dream!
b
NOTES
1
http://www.analog.com/library/analogdialogue/Anniversary/13.html
https://ewhdbks.mugu.navy.mil/VSWR.htm
2
This article can be found at http://www.analog.com/library/analogdialogue/archives/38-10/wideband_switch.html, with a link to a PDF.
Analog Dialogue Volume 38 Number 4
7
Support for the Designer—
Improved ADI Website Helps You
By Pam Aparo [pamela.aparo@analog.com]
Reza Moghimi [reza.moghimi@analog.com]
We have recently reconfigured the Analog Devices website
to make it easier to use. If you are an old friend, perhaps
you’ve already found that www.analog.com is friendlier than
ever. If you are new to our website, Welcome! As you explore
it, you’ll find much useful information, and you’ll have the
ability to interactively find and apply the best products for
your designs, using information listed under the two major
tabs on the Home Page: Products and Design Center.
There you will discover utilities to find the right product for your
application—and tools to help you use that product successfully.
Utilities that assist in product selection include parametric searches,
selection tables, and signal chains. Tools that assist in implementing
the products include models, interactive design tools, Analog Wizards,
SimPLL, and SimADC. These analog tools can be found in our
Design Center (www.analog.com/designcenter), which also
includes useful links to DSP tools, evaluation boards, and the
library of technical information readily available from ADI.
The most powerful product selection tool on the site is the
parametric search. When you know the specifications a device must
have, the parametric search tool is the fastest way to find products
to meet those requirements. The parametric searches allow you
to specify a great many performance characteristics of a device.
For example, the op-amp parametric search normally loads with
11 commonly used parameters—and 22 additional parameters
can be added to the table to narrow the search. To keep the table
from being too large, you can remove columns that are irrelevant
to your design. The links to the analog-product parametric search
tool can be found next to the search field on most pages of the
corporate site.
Parametric searches are currently available for
• Op amps
• DACs
• Multiplexers
• Linear regulators
• References
SENSOR
• ADCs
• Switches
• Interface products
• Supervisory circuits
Another product selection tool is the set of customizable selection
tables, which can be found in the Design Center under Parametric
Selection Tables, or under the Products tab. Selection tables are
used to search through a smaller subset of products. Most of the
selection tables on the Analog Devices site are interactive, allowing
products to be sorted according to the parameters most important
to the design. These tables let some parameters be selected with
drop-down options; others can be sorted using the up/down
arrows. For example, when using the interactive selection table
for comparators, click on customize table for access to customizable
parameters; select the logic level, then sort on the propagation
delay. To get more information about a specific listed product,
click on the model number to access the product page.
To find a part that ADI recommends for a specific application, a
signal chain is a good place to start. Signal chains can be found at
the top of the right-hand column of the Design Center. They are
classified by market, and then by application within that market.
The interactive signal chain for a selected application shows a
block diagram implementation of that application (see Figure 1).
The blue icons in the signal chain represent products that
can be furnished by Analog Devices. Clicking on a blue icon
will display a menu of product types that are applicable for that
block. In the digital-camera signal chain, for example, clicking
on audio amplifier will display the selections of audio amplifier,
output amplifier, and volume control. Clicking on any of these will
provide an interactive selection table of appropriate products. For
each selected product, a single click will add it to a list that may be
used for further in-depth study—or for ordering samples.
AMP
MUX
SENSOR
To use the parametric search, check the include parameter boxes
to use the desired parameters, or uncheck the boxes to ignore
those that are not of interest. Check the priority boxes to identify
the parameters that are more important. To optimize a search on
a specific parameter, type “best” in its query parameter field. For
example—when looking for the lowest-power op amp that meets
certain specs for headroom, supply voltage, and package—you
would type “best” in the quiescent current column and specify the
remaining requirements in the other columns. The search returns
a list of all parts that meet the requirements. If there are fewer
than 10 parts with the specs that are needed, the search will also
return parts that almost meet your needs. Clicking on the down
or up arrow for a given parameter will sort the list from highest to
lowest or lowest to highest. Any required specs that are not met by
a listed product will be highlighted in red. To get more information
about a specific listed product, click on the model number to access
its product page—which links to the data sheet.
AMP
ADC
ISOLATION
AMP
SYSTEM CONTROLLER PROCESSOR
AND FILTER PROCESSOR
PROCESS
SYSTEM VREF/
TIMING
VALVE CONTROL
MOTOR CONTROL
DIGITAL
POT
POWER
MANAGEMENT
AMP
DAC
SIGNAL
CONTROL
SYSTEM CONTROL
INTERFACE
ISOLATION
SUPERVISORY
THERMAL
MANAGEMENT
INDUSTRIAL MEASUREMENT/PROCESS-CONTROL SIGNAL CHAIN
Figure 1. A typical signal chain.
8
Analog Dialogue Volume 38 Number 4
In addition to helping find the appropriate products, the Design
Center has tools that can be used to shorten design time. Analog
Devices has a suite of models, online tools, downloadable tools,
and evaluation boards available. SPICE models are available for
many op amps, instrumentation amps, references, and analog
multipliers. ADI’s SPICE models can be used to closely replicate
transient and ac device performance. IBIS models, used to model
the input and output characteristics of a device, are available for
ADCs, DACs, and DSPs. Saber models are available for some
instrumentation amps.
the product families of IC manufacturers have combined to create
a need for this tool. This situation is further compounded by evershrinking product-design cycles, calling for ever-faster solutions.
ADI also offers interactive design tools to predict the behavior of
some devices. For many op amps and instrumentation amps, there
are error-budget and voltage-range calculators. The voltage-range
calculators are invaluable for working through an in-amp design in
which the internal nodes can become saturated. Equations in the
data sheet come to life when you can see the voltages on internal
nodes attempt to exceed the supplies. Differential amplifiers are also
more easily understood after using the online tool to see the effects
of common-mode and differential voltage inputs.
At first, the goal of the Analog Wizard was simply to narrow the
choice to a few possible candidate amplifiers or other ICs to perform
a specific function. But experience has shown that our role could
not be limited to simply recommending suitable ICs; many users
also requested that we specify the exact circuit configuration for
their specific requirement. In one instance, we had to recommend
an amplifier for a filter function, as well as to show the user how to
implement the desired 3rd -order filter with a set of active and passive
components. Thus, we needed to design and offer the complete
circuit solution to the user. The Analog Wizard is designed to
simplify the design task, providing many benefits to the user as
discussed below.
Other interactive design tools are used to help configure a device
for your application. For example, the ADF4110 has four 24-bit
registers. The register maps are included in the data sheet, and have
also been converted to an online tool. These register configuration
tools can be used to double-check your own calculations, saving
valuable design time. The website has register configuration assistants
for many of our ADCs, PLLs, and DDS products.
Free downloadable tools are also provided to simulate the
performance of PLLs and ADCs. The PLL simulation tool assists
in selecting a PLL and VCO, designing a loop filter, and predicting
the system’s performance. The ADC simulation is a virtual evaluation
board. By applying a behavioral model of the ADC to a collection of
characterization data, an engineer can quickly decide which ADC
will have the best performance with a specific input signal.
In addition to the interactive design tools that are specific to
individual products, we offer several useful utilities. These include
power dissipation, to predict how hot a device will become, conversion
between dBc and dBm, and a calculator showing the relationship
between SNR, THD, and ENOB.
Analog Devices, always aware of the needs for design assistance, has
from the get-go helped designers by offering seminars, extensive
publications—such as technical handbooks and in-depth data
sheets—expert applications engineers, and—more recently—
webcasts and web tools. Now, tools such as the Analog Wizard
enable us to be at your beck and call, providing a high degree of
applications support around the clock.
The Wizard’s operating approach is this: It asks the user a series of
application-specific questions. Albeit not needing to know about the
parameters of an amplifier or an ADC or microcontroller, the user
does need to be able to specify exactly what is required. For example,
the system designer needs to know the resolution requirements and
available power supply levels in the system (whether single or dual,
and their values). If at any point the user needs to understand the
reasoning behind the questions asked, or is unclear about some
of the terms used, the Wizard can help by providing links to
definitions, articles, references, and examples of manufacturers’
data sheets for explanations of the parameters and terms. The user
can change the suggested default response to a question in order to
see how it affects the solution.
Figure 2 shows an example of the types of questions asked about
photovoltaic mode of a photodiode.
Wizards are the colloquial name for analog design assistants, tools
that offer a more in-depth treatment of an application. Currently,
wizards are available for designing active filters and photodiode
circuits—and interfacing with bridge circuits. They are discussed
is some detail below. Other links at the design center include sample
code, a list of evaluation boards, and the DSP knowledgebase.
24  7 Amplifier Applications Support: Analog Wizards
“How do I find the best amplifier to use for a photodiode application?”
“What is the best amplifier to use in a bridge-type application?”
“What is the best amplifier to drive my ADC’s input circuit?”
These are some of the typical questions an application engineer
hears everyday. The Analog Wizard is a powerful tool that helps
you answer questions of this kind.
This web-based tool—a collection of analog design assistants—offers
circuit designs based on a user’s specific requirements and includes
a list of the suggested components. In the near future, extensions
to this collection will enable the Wizard to walk a user through the
elements of a complete signal chain. These web-based tools can be
bookmarked to be the starting destination for ADI website visitors
pursuing designs using our products.
The Analog Wizard is a timely response to the inexorably increasing
need for design assistance. The shrinking number of analog experts
in the world of systems design; the difficulty of finding a system
designer who can understand all the parameters of analog and digital
ICs; and the vast number of similar-but-different offerings within
Analog Dialogue Volume 38 Number 4
Figure 2. Typical Analog Wizard questions.
Based on the user’s answers, the Analog Wizard calculates and
defines its own search criteria for the Analog Devices product
database. It then enters these search criteria into the product
database, interrogates different part types that are suitable and
relevant to the application, and suggests a number of them. The
suggested parts are sorted based on required precision, temperature
range, package options, and pricing. This listing and ranking might
be presented in different ways, depending on the application. If
the Wizard cannot meet all the imposed search criteria, it will still
provide a close suggestion—with the failed parameters highlighted
in red—and will state a good reason for the failure. Figure 3 shows
a typical amplifier listing called up by a specific task.
9
In addition, the wizard allows the user to enter and compare a
generic part number (for a part that may already be on hand)
against the suggested parts. Further, the user can link to the
product page of the selected device from the solution page.
The maximum benefits from the wizard depend on the application
at hand. For instance, in the case of the Photodiode Wizard, the
user can look at the noise spectrum of the solution suggested. In
addition, it provides the theoretical and actual signal-to-noise ratio
(SNR). Figure 6 shows a typical noise spectrum for the circuit
shown in Figure 4.
Figure 3. Suggested amplifiers for a photodiode application.
Besides the suggested ICs (usually up to 5 different device
types), the user also receives a bill of material (BOM) and a
circuit schematic representing the designed solution, as shown
in Figure 4.
Figure 4. Typical solution circuit with bill of material.
The Analog Wizard provides many more benefits. It will provide
the PSpice model for ADI’s IC device when available. It will also
generate the circuit solution’s netlist. Users can save or cut-and-paste
this information into their PSpice model environment for further
analysis and evaluation in a theoretical world.
The BOM shows the theoretical values of the passive components
used in the circuit. Finding components in the real world that match
the theoretical values will be very costly and time consuming, if
not impossible, so the Analog Wizard allows the user to access
the error analysis tool. In this environment, the user can introduce
tolerances for resistance and capacitance values as shown in the
schematic of Figure 5 and perform further error analysis on the
suggested configuration.
Figure 5. Error analysis tool.
Figure 6. Typical noise spectrum for
a photodiode application.
To simplify the task of getting user feedback, acquiring data on
customer satisfaction, and providing any needed support, there
are links for feedback and to request customer support on all
pages of the Wizard.
The photovoltaic mode of photodiode application was picked
for development due to its popularity and the number of
questions asked by users. This could be followed in due course
by development of a wizard to handle photoconductive circuit
applications. The second module of Analog Wizard was the
Bridge Wizard. The third module released was the Filter Wizard.
The future of Analog Wizards looks very promising. There are
many other persistently recurring topics, such as A/D converter
drive amplifiers, currently handled by our application engineers
that could be dealt with in mutually beneficial fashion by these
design-assistant, or “Wizard,” modules. Just to contemplate a
signal chain, such as that shown in Figure 1, is to get a feeling
for the opportunities in product and system design.
Before we leave the subject, we must return to real-world
considerations and emphasize that these design assistants
are just that. They can speed up the process and relieve the
design engineer or technician of the arduous work of routine
calculations—but they will never relieve the designer of
the burden of engineering judgement. The list of real-world
considerations beyond those handled by these simple “Wizards”
is impressive: For example, the ever-increasing environment
of RF energy as a source of interference must be dealt with;
considerations of layout, grounding, strays, and parasitics
will always be with us; and questions relating to the ambient
physical environment, as it affects—and is affected by—the
design always require attention.
Nevertheless, these tireless design assistants exist on the website
to provide applications support around the clock. They are a
resource that provides many benefits to designers. They are selfguiding and helpful; they ask a series of questions in familar
application-oriented language (rather than the language of IC
parameters); they are helpful in the part selection process. They
can offer the elements of a complete solution, including, as shown,
a bill of materials and Pspice netlist; and they provide links to
further error analysis and evaluation tools and materials—and
ultimately to human assistance. Yet users can explore the
possibilities at their own convenience, with powerful help toward
designing the best solution using off-the-shelf parts.
b
This article can be found at http://www.analog.com/library/analogdialogue/archives/38-12/wizard.html, with a link to a PDF.
10
Analog Dialogue Volume 38 Number 4
Designing Efficient, Real-Time
Audio Systems with VisualAudio™
By Paul Beckmann [paul.beckmann@analog.com]
Vincent Fung [vincent.fung@analog.com]
The VisualAudio design and development environment is a new
software tool for designing and developing audio systems. Its
real-time-architecture is especially well-suited to the challenges
of audio product development. This article briefly introduces
VisualAudio, and then describes its framework, audio modules,
and application to audio product development.
Audio Product-Development Challenges
Today, audio system developers are faced with the increasing need to
design complex audio systems—especially home audio/video receivers
(AVRs) and automotive infotainment systems—both quickly and
cost-effectively. Why?
• The number of discrete audio channels in normal use has grown
from 2 to 4 to 5.1—and most recently—to 7.1.
• The number of distinct, and sometimes competing, multi-
channel audio formats has been increasing rapidly—to include,
among others, Dolby® Pro Logic®, Dolby Digital, DTS® 5.1,
Dolby Digital Surround EX™, and DTS-ES®.
• Products must interface with digital networks, such as the Media
Oriented Systems Transport (MOST®) bus, requiring network
stacks, content encryption and decryption, and sample-rate
conversion—all within the audio processor.
• Consumers have come to expect sophisticated post-
processing features, such as spatialization, automatic
equalization, and bass management—in both top-of-the-line
and mainstream products.
To deal with these factors, developers are turning to digital signal
processors (DSPs), because their programmability allows systems
to be customized for specific market niches and applications.
The SHARC ® Processor family from Analog Devices (ADI)
is particularly well-suited for this task, since it offers features
such as large internal memory, floating-point precision, and
high-performance computation units. The recently announced
third-generation SHARC processors take this one step further by
integrating additional features that are specifically introduced to
facilitate audio product design. These features include hardware
sample-rate converters, encryption and decryption, a sophisticated
digital audio interface, and an on-chip ROM containing multiple
audio decoders.
The historical challenge faced by DSP users has been the
development of software that makes optimum use of processor
clock cycles and efficient use of memory. The long-used and
laborious approach of hand-coding audio signal processing
algorithms in assembly language has become less and less viable.
This is particularly true when a large portion of the required effort
goes into creating standard “checklist” and “me-too” functions
instead of focusing on differentiating the product with value-added
features. A better approach to developing audio product software
was required.
To fulfill this need, ADI has developed a graphical environment—
VisualAudio—as an aid to designing and developing audio systems
that use the SHARC processor family. VisualAudio provides audio
system developers with most of the software building blocks—
together with an intuitive, graphical interface, shown in Figure 1—for
designing, developing, tuning, and testing audio systems.
Analog Dialogue Volume 38 Number 4
Figure 1. Example of VisualAudio graphical-interface screens.
VisualAudio comprises a PC-based graphical user interface (GUI, the
graphical tool), a DSP kernel (the framework), and an extensible
library of audio algorithms (audio modules).Working in conjunction
with ADI’s VisualDSP++™ integrated development and debugging
environment (IDDE), VisualAudio generates product-ready code
that is optimized for both speed, in millions of instructions per
second (MIPS), and memory usage. By simplifying the process of
developing complex digital signal-processing software, VisualAudio
reduces development cost, risk, and time. As a result, audio-system
developers are able to focus on adding value by differentiating their
audio products from the competition.
At the core of VisualAudio is a real-time software architecture
that handles audio I/O and post-processing. In order to be viable,
the generated DSP code must be efficient in terms of MIPS and
memory, and be flexible enough to handle a variety of audio product
categories.The VisualAudio real-time architecture is described below,
first the framework and then the audio-processing modules.
The Framework
The framework is the portion of the DSP code that handles system
initialization, audio I/O, bit stream detection 1, instantiating
and calling audio decoders, and communication with the host.
VisualAudio provides its users with examples of frameworks
for AVRs and automotive audio systems. By writing platformspecific drivers, VisualAudio users can customize many aspects
of the framework to address specific product requirements. In
some cases, ADI will also make the framework source code
available to VisualAudio users if internal changes are necessary
for optimum performance.
Audio products have specific requirements that govern the design
of the framework. Each audio product has two primary functions:
(1) real-time audio processing, and (2) control of this processing. The
time scales for these two functions are vastly different. The real-time
processing (with all internal operations completed) must occur at the
sampling rate, otherwise there will be unacceptable pops and clicks
in the output audio. The control functionality can occur at a much
slower rate, 10 Hz to 100 Hz, and still be acceptable. The bulk of the
MIPS usage thus occurs within the real-time processing, while the
bulk of the software complexity is within the control functions. In order
to simplify product design and development, VisualAudio separates
the real-time and control functions into separate threads. Efficiency
is achieved by hand-optimized, real-time audio processing modules,
while the complexity of control code is managed by allowing the
developer to write it in C and run it in a separate thread.
1
The bit stream detector, used with digital inputs such as an S/PDIF interface, monitors
the incoming data stream and distinguishes between uncompressed PCM audio and
compressed audio.
11
Traditionally, two different approaches to audio processing have
been taken. In stream processing, audio samples are processed one
at a time as they arrive, while in block processing several audio
samples are buffered and then processed as a group. Each method
has distinct advantages and disadvantages. Stream processing is
efficient in terms of data memory, since no buffering of audio data
is required. The major limitation of stream processing is that the
overhead of multiple function calls cannot be tolerated. This forces
the audio processing code to be written in-line, usually in assembly
language. Such code is difficult to modularize and maintain.
Block processing requires additional buffering memory for I/O
and scratch memory. Typical block sizes are in the range of 32
to 256 samples. Since many samples are processed at once, the
overhead of function calls is amortized over a large number of
samples. This leads to MIPS-efficient implementation—at the
expense of additional memory—but is preferred, since structured
programming methodologies may be employed. Block processing
is also a natural fit to audio decoders that generate blocks of audio.
For example, both Dolby Digital and DTS decoders generate
audio in 256-sample blocks.
Block processing, the approach used by VisualAudio, has several
additional advantages. All audio I/O within VisualAudio is doublebuffered and managed using direct memory access (DMA). The
processor receives an interrupt once per block—not once per
sample—resulting in much less interrupt overhead than with
stream processing. Also, by utilizing the chained DMA capabilities
of the SHARC processors, double buffering is managed by the
DMA controller, significantly increasing the allowable latency
when servicing an audio input/output (I/O) interrupt.
The VisualAudio framework delivers audio to the post-processing
network in blocks. Certain restrictions are placed on the block
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size. First, it must be an even number, due to the single-instruction,
multiple-data (SIMD) behavior of some audio modules. Second,
the minimum block size is eight samples—due to pipelining within
some audio modules. Finally, in systems with an audio decoder, the
post-processing block size must be a factor of the decoder block
size. For example, with Dolby Digital, possible block sizes are 8,
16, 32, 64, 128, and 256 samples.
Audio I/O and buffering can be seen within the example of a
VisualAudio automotive framework, shown in Figure 2. Audio
arrives from either the MOST network or from A/D converters,
and is separated into multiple streams. The primary entertainment
stream is generated by the DVD player, and additional monaural
streams are produced by the telematics system or by chimes.
The DVD data first undergoes digital-transmission copy-detection
(DTCP) decryption, and is then fed to the bit stream detector.
The output of the bit stream detector is packed into blocks; when a
complete frame of data is available, the audio decoder is executed.
The DVD player generates its own sampling rate, which is distinct
from the sampling rate used by the post-processing. Thus, the
output of the audio decoder must pass through an asynchronous
sampling-rate converter. This block converts all input data streams
to a fixed output sampling rate. At this point, the audio postprocessing is executed at a fixed block size of 32 samples. As a final
step, the audio channels are fed to D/A converters or returned to
the MOST network.
The automotive framework contains multiple audio decoders,
only one of which is active at a time. To reduce decoder memory
requirements, VisualAudio manages a decoder memory pool
that is shared among all possible decoders. The output of the
decoder is fed to the post-processing network; and this drives
the D/A converters.
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Figure 2. Audio flow within the VisualAudio automotive framework.
12
Analog Dialogue Volume 38 Number 4
VisualAudio uses a simple interrupt driven kernel to manage multiple
threads. For example, the sample automotive framework contains a
total of six threads. From highest to lowest priority they are:
Host communication—exchanges messages with the host—
typically via SPI. Messages are buffered and interpreted within
the DSP user control code (described below).
Audio transmit interrupt—triggered by the serial port
interrupt. Manages output DMA to the DACs—and formats data
to be returned to the MOST network. Triggers audio processing
within User Interrupt 0.
Audio receive interrupt—separates audio into distinct
streams. Performs DTCP decryption and packs encoded data
into frames. When appropriate, triggers the audio decoder within
User Interrupt 1.
Audio processing (User Interrupt 0)—performs postprocessing on a 32-sample block. The bulk of processing occurs
within this thread.
Audio decoder (User Interrupt 1)—executes the audio
decoding function.
DSP user control code (UCC)—runs whenever there are no
interrupts active and serves as the main loop of the application. The
user product’s control functionality occurs within this thread.
Typical thread activity within the sample AVR framework is shown
in Figure 3. Each horizontal slice represents a different thread.
The audio I/O, decoder, and the post-processing are run at regular
intervals, and the UCC runs at the lowest priority. Note that the
UCC might not be run for several milliseconds—while the processor
is busy with audio processing.
HOST
COMMUNICATION
AUDIO TRANSMIT
INTERRUPT
or to query the state of the audio processing (e.g., “Is the system
limiting?”). The VisualAudio framework handles some commands
internally, while the remainder are passed to the UCC. At each
point in time, there can be only one pending message between the
host and the DSP—and the DSP must send an acknowledgement
after each command has been processed.
Notifications are generated asynchronously by the framework and
occur under several conditions. The first notification occurs during
system initialization—before any real-time processing is enabled.
System- or application-specific initialization may be done at this
time. A second notification, generated periodically at a rate of
approximately 200 Hz, is used for control of the real-time audio
processing, for example, automatic gain-control (AGC) calculations
and updates. A final class of notifications is generated by the
audio decoders in response to changes in the encoded bit stream.
Notifications of this type occur when the sampling rate changes,
the number of input channels changes, or if a cyclic redundancycheck (CRC) error is detected, in the incoming bit stream. These
notifications allow the UCC to make appropriate changes in the
audio processing.
Since the UCC is pre-empted by real-time audio processing, it may
not be executed until several milliseconds have gone by—as shown
in Figure 3. VisualAudio includes several features that simplify
writing a UCC that will be constantly subjected to interrupts.
First, since the host-communication interface only allows a single
host message to be pending between the host and DSP, there is
no danger of overflowing a message buffer, or of host messages
overwriting each other. Another feature is a notification queue, in
which notifications of the same type overwrite each other. For
example, if two sampling-rate notifications are generated closely
spaced in time before the UCC is executed, then the UCC will only
receive the second notification—the final sampling rate. Also, since
there are a finite number of notifications, the notification queue is
necessarily of finite length.
The UCC must also be carefully written for updating certain
audio module parameters. Some module parameters, such
as infinite impulse-response (IIR) filter coefficients, must be
updated automatically without being interrupted by the
audio processing.
AUDIO RECEIVE
INTERRUPT
AUDIO PROCESSING
(USR0)
AUDIO DECODER
(USR1)
USER CONTROL
CODE
TIME
Figure 3. Thread activity within the VisualAudio AVR
framework. The threads are ordered from high priority
(at the top) to the lowest priority (at the bottom).
VisualAudio partitions control functionality between the host
microcontroller and the DSP. This partitioning is arbitrary, and can
even support systems without a dedicated host microcontroller. As
described above, the UCC executes at lowest priority—using any free
cycles not consumed by the interrupt handlers. The UCC is called
periodically by the framework with messages and notifications.
Messages are commands sent from the host microcontroller to the
DSP. These commands are used to control audio processing (e.g.,
“Set the volume to –20 dB,” “Set the bass tone-control to +3 dB”)
Analog Dialogue Volume 38 Number 4
Each VisualAudio framework has an associated XML platform
file that describes the capabilities of the target platform to the
VisualAudio application, and also contains a list of the source, object,
and library files needed to build the executable. Software design
and development usually begins on a readily available evaluation
or development platform, such as an EZ-KIT Lite evaluation kit
from ADI, and then migrates to the actual target hardware once it
is complete. VisualAudio’s Change Platform Wizard automates the
process of migrating software between hardware platforms.
The Audio Modules
VisualAudio contains a library of approximately 100 audioprocessing modules that have been optimized for the SHARC
processors. The modules, categorized by function, include volume
controls, tone controls, filters, mixers, etc.—enough functional types
to develop a wide variety of audio products. These standard audio
modules can be augmented by custom modules implementing
proprietary post-processing features.
13
Figure 4 demonstrates the efficiency of block processing for a 10th
order IIR filter—implemented as a cascade of five biquad filters.
The number of operations per sample is plotted as a function
of the block size. Most efficiency gains have been realized by a
64-sample block, with diminishing returns for larger blocks. For
this filter, the core inner loop contains 21 multiply-accumulates
(MACs) per sample; but by the use of SIMD instructions that
operate on two pieces of data simultaneously, the loop is reduced
to roughly 15 cycles.
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VisualAudio creates a separate data structure for each instance
of an audio module within the post-processing layout. All data
structures share a common 5-word header that describes the
module’s run-time interface to the framework. This is followed
by module-specific parameters and state variables. For example,
the type declaration of the data structure for the scaler module
described above is:
typedef struct
{
AMF_Module b;
float amp;
} AMF_Scaler;
// Common 5 word header
// Instance specific variable
The audio processing functions within VisualAudio follow a
uniform calling sequence. Each function is called with three
arguments: a pointer to the instance data structure, a pointer to
an array of input and output buffer pointers (input buffers first,
then output buffers), and an integer specifying the block size.
Continuing the example of the scaler, its real-time processing
function is defined below.
��
��
and tunable parameter, allowing it to be modified at design time,
and later, during real-time tuning. This monaural fixed-gain
example illustrates one of the many variable types that can be
described by the VisualAudio XML format.
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���
��������������������
Figure 4. Processing efficiency in an IIR filter as a
function of the block size.
Each audio module is described to the VisualAudio application
by an associated XML file. This file contains a description of the
module’s in-memory data structure, memory allocation rules,
input- and output audio channel list, high-level interface variables
(shown on the module’s graphical representation, or inspector), and
design equations. The XML language consists of tags that label
structured data. For example, consider a simple audio module
that scales a monaural signal by a fixed gain. This module would
contain a single render Variable—amp—that specifies the gain to
apply. Within the module’s XML file, the variable amp is described
by the XML code shown below:
<renderVariable>
<description type=”string”>amplitude applied
to input</description>
<name type=”string”>amp</name>
<float>
<min type=”float” modify=”true”>-1.0</min>
<max type=”float” modify=”true”>1.0</max>
<default type=”float”>1.0</default>
</float>
<usage type=”string”>parameter</usage>
<designSettable type=”bool”>true</
designSettable>
<tunable type=”bool”>true</tunable>
</renderVariable>
The description tag provides a short summary of the variable. The
name tag indicates the variable name within the data structure.
The variable is described as being floating-point, having a
default range of [–1 +1] that can be modified, and a default
value of 1.0. VisualAudio will treat this variable as a design-settable
void AMF_Scaler_Render(AMF_Scaler *instance,
float ** buffers,int blockSize) {
int i;
float *in = buffers[0];
float *out = buffers[1];
float amp = instance->amp;
for (i=0; i<blockSize; i++) {
out[i] = in[i] * amp;
}
}
Note that this example is written in C to clarify its description.
The actual render function included with VisualAudio is written
in optimized assembly language for speed of execution.
SUMMARY
The complexity of digital signal processing software within audio
products continues to increase. To address this, Analog Devices
has developed VisualAudio, a graphical audio system design and
development environment. VisualAudio simplifies audio product
design and development by providing developers with many of the
key software building blocks found in audio systems, managed
with an intuitive graphical interface. The VisualAudio real-time
architecture is flexible enough to support a variety of product types
and is also efficient in terms of MIPS and memory. The overall
VisualAudio tool-chain jump-starts product development; reduces
development cost, time, and risk; and allows engineers to focus
on innovation and product differentiation.
b
More about VisualAudio
The VisualAudio design and development environment is expected
to become available in November 2004, for use in conjunction with
VisualDSP++ (release 3.5) for the SHARC Processor family. To
take a VisualAudio test drive or learn more about VisualAudio and
other Analog Devices development tools, visit: www.analog.com/
processors/tools.
This article can be found at http://www.analog.com/library/analogdialogue/archives/38-11/visual_audio.html, with a link to a PDF.
14
Analog Dialogue Volume 38 Number 4
PRODUCT INTRODUCTIONS: VOLUME 38, NUMBER 4
Data sheets for all ADI products can be found by entering the model
number in the Search Box at www.analog.com
October
Amplifier, Difference, single-supply, high common-mode voltage range .. AD8202
Audio Processor, 28-bit multichannel SigmaDSP® .............................. AD1940
Digital Downconverter, 150-Msps wideband ..................................... AD6636
DAC, TxDAC+® , 16-bit, 200-/500-Msps ............................................ AD9786
Gamma Reference, 12-channel, includes regulator and VCOM buffer ..... ADD8707
Gamma Buffer, 18-channel, includes regulator ................................ ADD8709
Gyroscope, single chip with signal conditioning,
measures yaw rates up to 75/s ...................................................... ADXRS401
Laser Diode Driver, dual-loop, operates from 50 Mbps to 3.3 Gbps .. ADN2870
Switch, CMOS, wideband, dual SPDT, provides
36-dB isolation at 1 GHz ............................................... ADG936/ADG936-R
November
Accelerometer, 5-g, small, thin package ........................................ ADXL320
ADC, Successive-Approximation, 16-bit, 100-ksps ......................... AD7683
ADC, Successive-Approximation, 4-channel,
10-/12-bit, 188-ksps, I2C® interface ....................................... AD7993/AD7994
ADC, Successive-Approximation, 8-channel,
10-/12-bit, 188-ksps, I2C interface ........................................ AD7997/AD7998
Amplifier, Variable-Gain, quad, low-noise, low-cost .......................... AD8335
Amplifier, Operational, CMOS, dual, precision, low-noise,
low-distortion, 50-MHz ..................................................................... AD8652
Controller, Synchronous Buck, 2-/3-/4-phase for Intel processors .... ADP3181
Controller, Synchronous Buck, 2-/3-/4-phase for AMD processors .. ADP3186
Frequency Synthesizer, PLL, includes voltage-controller oscillator .. ADF4360-8
Gamma Buffer, 18-channel, mask-programmable,
includes voltage regulator ............................................................... ADD8708
Line Driver, ADSL, upstream, rail-to-rail outputs .............................. AD45048
Touch-Screen Controller .................................................................. AD7877
Video Decoder, 9-bit, Multiformat SDTV ...................................... ADV7181B
Video Decoder, 10-bit, Multiformat SDTV .................................... ADV7183B
Video Decoder, 12-bit, Multiformat SDTV .................................... ADV7189B
December
ADC, Successive-Approximation, 16-bit, 100-ksps,
true-differential inputs ....................................................................... AD7684
ADC, Successive-Approximation, 16-bit, 250-ksps,
pseudo-differential inputs .................................................................. AD7694
ADCs, Sigma-Delta, 3-channel, 16-/24-bit, low-noise,
low-power ............................................................................ AD7792/AD7993
ADC, Sigma-Delta, 6-channel, 24-bit, low-noise, low-power .............. AD7794
ADCs, Successive-Approximation, 8-channel,
12-/10-bit, 1.5-Msps ............................................................. AD7938/AD7939
ADC, Successive-Approximation, 8-channel, 12-bit, 625-ksps ....... AD7938-6
Amplifier, Difference, single-supply, high common-mode
voltage range ..................................................................................... AD8203
Amplifier, Instrumentation, precision zero-drift, rail-to-rail
inputs and outputs ............................................................................. AD8230
Amplifier, Operational, dual, precision zero-drift,
rail-to-rail inputs and outputs ............................................................. AD8629
Amplifier, Operational, low-cost, high-speed, rail-to-rail output ... ADA4851-1
Battery Charger, Linear, 1.5-A for single-cell lithium-ion battery ..... ADP2291
DAC, Voltage-Output, 40-channel, 14-bit, 3-V/5-V supply ................. AD5384
DACs, Multiplying, dual 8-/10-/12-bit, wideband,
parallel interface ..................................................... AD5428/AD5440/AD5447
Digital Upconverter, multichannel, VersaCREST™
crest-reduction engine ....................................................................... AD6633
Multiplexer, CMOS, quad 2:1, 300-MHz bandwidth,
HDTV audio/video applications ........................................................ ADG794
RF Splitter, Active ...................................................................... ADA4302-4
Sequencers, Power Supply, supervise and monitor up to 10 supplies ... ADM106x
Supervisory Circuits, Microprocessor, include watchdog
and manual reset ............................................................................ ADM63xx
Supervisory Circuits, Microprocessor, include watchdog
and manual reset ............................................... ADM823/ADM824/ADM825
Switch/Multiplexer, CMOS, SDPT/2:1, 0.5-ohm on-resistance ......... ADG839
Switch/Multiplexer, CMOS, dual 2-channel, 0.5-ohm on-resistance .. ADG884
Transceivers, RS-485/RS-422, half-duplex, low-power,
slew-rate-limited outputs .................................................................. ADM483
Transceiver, RS-485, half-duplex, isolated ....................................... ADM2483
Transceivers, RS-485/RS-422, half- and full-duplex,
slew-rate-limited outputs ................................................................ ADM485x
Transmitter, FSK/GFSK/OOK/GOOK/ASK, multichannel,
ISM band ....................................................................................... ADF7012
Vector Multiplier, RF/IF ................................................................. ADL5390
Video Encoders, Multiformat, include six 12-bit
noise-shaped video DACs ................................................ ADV7320/ADV7321
Analog Dialogue Volume 38 Number 4
AUTHORS
Pam Aparo (page 8) received a BSEE
degree from the University of Connecticut.
After working in manufacturing and medical
instrumentation, Pam joined ADI as an
applications engineer and has contributed
to ADI’s website redesign. She recently
transitioned to a new role as the Worldwide
Distribution Training Manager. In her spare
time, she enjoys tackling home improvement
projects and training her dogs.
Paul Beckmann ( page 11) is an
independent consultant working with
the Analog Devices Audio Rendering
Technology Center (ARTC) in San Jose,
CA. He specializes in audio system design
and development, including algorithms,
efficient implementations, and overall
system architecture. Paul spent nine years
at Bose Corporation in R&D and product
development. Paul received SB, SM, and
Ph.D. degrees in Electrical Engineering, all from MIT.
Theresa Corrigan ( page 3), an
applications engineer at ADI’s facility in
Limerick, Ireland, provides applications
support for switch/multiplexer products
and for general-purpose digital-to-analog
converters (DACs). Theresa, who holds a
BSc. (Hons) in Experimental Physics from
National University of Ireland, Galway,
joined A nalog Devices in 2001. Her
interests include playing soccer, reading,
and traveling.
Vincent Fung (page 11) has been the
Product Marketing Manager for Platform
Tools Group’s Audio Rendering Technology
Center in Santa Clara, California, since
2002. Prior to joining Analog Devices,
he has held key product-marketing and
business- development positions at both
established-high-technology companies
and digital-audio distribution start-ups. He
holds both Bachelor and Master degrees in
Electrical Engineering, as well as an MBA in Marketing, from
Columbia University.
Reza Moghimi ( page 8), an A DI
Applications Engineer in San Jose, received
a BSEE from San Jose State University in
1984 and an MBA in 1990—and has also
received a number of on-the-job certificates.
He has worked for Raytheon Corporation,
Siliconix, Inc., and Precision Monolithics,
Inc. (PMI) —which was integrated with
Analog Devices in 1989. At ADI, he has
served in test-, product-, and projectengineering assignments. He has written many articles and
design ideas—and has given presentations at technical seminars.
His hobbies include travel, music, and soccer.
15
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